1. Field of the Invention
This invention relates to a new process for the production of surfactants and surfactant mixtures based on fatty alcohol ether sulfonates which enables this industrially interesting class of surfactants to be economically obtained.
2. Discussion of Related Art
Enhanced oil recovery, in which the quantities of residual oil retained in the reservoir by viscosity and capillary effects in the pore space are made at least partly recoverable, is acquiring increasing interest. Many types of process have been proposed for enhanced oil recovery, including for example polymer flooding, alkali flooding, thermal processes or solution flooding.
The present invention is concerned with the process known as surfactant flooding. Whereas, in the fresh oil reservoir, the oil is present as continuous phase in the rock pore space, the oil phase disintegrates with increasing primary and secondary recovery into individual discrete droplets which are retained in narrow pores under the effect of the high interfacial tension. Overcoming the capillary forces either requires extremely high pressures or a considerable reduction in the interfacial tension between water and oil using suitable surfactants. In each individual case, this reduction depends to a very large extent on the reservoir temperature, the salinity of the reservoir water and the composition of the oil. The desired so-called middle phase micro-emulsion--a central third phase between the heavier salt water phase and the lighter oil phase--is only formed through adaptation of the particular surfactant mixture and under the conditions of the extreme reduction in interfacial tension. The formation of the middle phase micro-emulsion is crucially important to the recovery of residual oil.
Suitable surfactants have to satisfy various requirements over and above their ability to reduce interfacial tension to a considerable extent. For example, they must not form any deposits in the flood water or formation water because otherwise there would be a danger of irreversible blockage in the reservoir. They should be adsorbed to the rock to only a minimal extent, if at all. They should be stable under reservoir conditions, remaining stable for periods of 1 to 3 years for distances between the injection probe and the recovery probe of 50 to 300 m and flooding rates of approx. 0.3 m/d.
Ether sulfates, ether carboxylates, ether sulfonates and ether phosphates in particular have been proposed as surfactants for use in high-salinity reservoir waters. Ether sulfates and ether phosphates are readily obtainable on an industrial scale, but lack stability to hydrolysis. Ether sulfonates and, more particularly, alkyl ether sulfonates have shown particularly interesting properties in screening tests. They combine high electrolyte compatibility with high stability to hydrolysis at high temperatures. In particular, however, they also show the appearance of the desired three-phase states in the oil/water/surfactant systems with a broad middle phase micro-emulsion range and a pronounced reduction in interfacial tension. Mixtures of these surfactants with other surfactants and/or so-called co-solvents open up technically interesting possibilities in the field of enhanced oil recovery.
Numerous syntheses have been proposed for the preparation of ether sulfonates, cf. for example the Article entitled "Fettalkoholethersulfonate fur die tertiare Erdolforderung (Fatty Alcohol Ether Sulfonates for Enhanced Oil Recovery)" in Fette-Seifen-Anstrichmittel, 1985, 382-385 and the literature cited therein. The problem addressed by the present invention was to provide a technologically simple process which would enable this interesting class of surfactants to be economically produced and which, in particular, would provide as a direct reaction product a surfactant-containing multicomponent mixture which would have valuable properties as such for the stated application. This primary surfactant mixture would consist of the above-mentioned alkyl ether sulfonates as the desired main component and, in addition, would contain nonionic fatty alcohol ethers forming the basis of the ether sulfonates and, optionally, so-called co-solvents based on synthetic and/or natural alcohols.